1. Field of the Disclosure
The present disclosure relates to an Acme screw/nut set, and more particularly to an Acme screw/nut set having a modified thread design.
2. Background of the Art
Drive mechanisms for different applications utilizing a lead screw as a driver usually use a standard Acme screw class G or C. A standard centralizing Acme screw/nut set class C has defined tolerances per ANSI B1.8 specification. Those tolerances provide very low clearances between the thread of the nut and the thread of the screw. For example: a 1½-5 ACME thread class 2C has the following clearances:
for a major diameter a radial clearance is Rmin=0.0012″ to 0.0098″ and
for a pitch diameter an axial clearance is Amin=0.0025″ to 0.14″.
The clearances are extremely low for the lower tolerance range. Therefore, a problem arises when using dissimilar materials with significantly different thermal expansion coefficients (e.g. steel and nylon). That is, the clearances will close quickly when the temperature of the joint increases due to the heat generated by friction between the components in the drive mechanism. The problem is especially prevalent in a design where the nut is confined in a rigid housing, thereby restricting radial expansion and allowing expansion of the nut material mainly in the inward direction. The lack of clearance between the screw and the nut may initially result in a grinding noise and finally in seizing the motion of the joint.
The following example is illustrative:
Assume the following materials and dimensions:                Acme screw D=1½″ major diameter and P=0.200″ made of carbon steel        Acme Nut (modified) of same basic thread with O.D.=1.125″ and 2.5″ long made of nylon 6 with a thread engagement L=2.312″        Nut housing made of aluminum with bore B=2.125″ dia.        Carbon steel has a coefficient of thermal expansion                    CTEs 8.1*10 E-6 in./in. ° F.                        Nylon 6 has a coefficient of thermal expansion                    CTEn 0.45*10 E-4 in./in. ° F.                        Aluminum housing has a coefficient of thermal expansion                    CTEh 13.1*10 E-6 in./in. ° F.                        
For the screw/nut pair in this example, it would take a temperature increase (ΔT) of 17° F. from the ambient temperature to close the gap of 0.0012″.
The relevant calculations for determining the effect of a temperature rise on the gap are as follows:
The nut material would expand radially inward (Rn) (assuming zero outward expansion allowed by the housing)Rn=ΔT*CTEn*D=17*0.45*10*E-4*1.5=0.0011475″
The screw material would expand radially outward (Rs)Rs=ΔT*CTEs*D=17*8.1*10*E-6*1.5=0.00020655″
The housing material would expand radially outward (Rh) (allowing the nut to expand outward the same amount). However, the expansion of the housing material is to a lesser degree than the expansion of the screw and the nut, due at least in part to the fact that the temperature of the housing material rises only approximately 30% of the temperature rise of the two other components (based on taken measurements).Rh=0.3*ΔT*CTEh(aluminum)*B=0.3*17*13.1*10*E-6*2.125=0.00014187″
The total expansion (R) of the joint in a radial direction may be calculated as follows:R=Rn+Rs−Rh=0.0011475+0.00020655=0.00014197=0.001212″The temperature of the Acme screw/nut surface may be subjected to temperatures up to 200° F. based on the material specification of nylon 6, for example, for a high load condition. Accordingly, undue friction and potential binding of machine parts may occur. The screw/nut design of the present disclosure may ameliorate such occurrences.